This is part of my (Re)Read CSL in Publishing Order project. It should have preceded Pilgrim's Regress; whoops. The volume I read was a re-released edition, with a preface from Lewis looking back at his earlier work. He notes that he did not so much compose the storyline as it came to him (at age 17): a man who begets a beast, which, on killing him, becomes a god.
I read that very summary in the preface, but forgot it entirely as I read the poem, because the bulk of the poem doesn't spend as much time on that. There's a canto of Dymer leaving the Perfect City (a now-typical sort of picture of an authoritarian society) for some wild and beautiful country; he eventually comes upon a building, open to his entry, with lovely music and rich clothes and a rich feast and, eventually, a beautiful woman to sleep with. When he wakes, she has disappeared. The bulk of the poem, to my reading, is his search for her. He instead finds a shrivelled old woman; a dying soldier who curses Dymer's name (not knowing with whom he speaks), a deadly magician, and, finally, a man hunting a brute. Dymer realizes this brute is his offspring, volunteers to face it, and (in one, perhaps two verses out of ~270) is struck down. The brute turning god had left my mind entirely by the time I got to it. The way that transformation is written reminded me of nothing so much as the end of Moana, where Te Ka is restored to the lovely Te Fiti. The denouement is just as sudden, too: there's a new god and the poem ends. Who knows what fallout occurs thereafter.
As usual, reading Lewis's earlier writing diverts me; the tropes or images I know he will use later on strike me with their echoes: the fine clothes of Cairn in The Magician's Nephew (and at the end of That Hideous Strength); the feast on the star's island in Voyage of the Dawn Treader; the man's journey in Pilgrim's Regress and Out of the Silent Planet and Perelandra.
Also, while other readers are not generally impressed by CSL's narrative poetry, I found myself pleased with the regularity of his meter, the generally unforced quality of his rhyme, and his skilled enjambment. It is sometimes beautiful, and almost always clear in drawing the scene.